Self-extensible fibers are known. For instance, JP 43-28262B proposes a process of producing a self-extensible polyester fiber tow or staple fiber, comprising passing a tow of shrinkable polyester filaments having a birefringence of at least 0.15 and a crystallinity less than about 35% through a stuffer box crimper and, at the same time, heating the filaments in the crimper with steam or water at 85° C. to 250° C.
JP 2000-96378A proposes a process of producing a self-extensible yarn which is also a polyester fiber, comprising moist-heat treating an undrawn, partially oriented polyester multi-filament yarn with no tension applied at a temperature near which the yarn exhibits the maximum dry heat shrinkage stress.
These self-extensible fibers or yarns are designed for use as multi-filament yarns or filament mixed yarns and are not contemplated for application to nonwoven fabrics, particularly thermal-bonded nonwoven fabric. The self-extensible fibers cannot be used alone to make thermal-bonded nonwoven fabric because they are not per se heat fusible. When they are used to make thermal-bonded nonwoven fabric, they must be combined with heat fusible fibers, which makes the process complicated and is economically disadvantageous. Moreover, in order for the thermal-bonded nonwoven fabric to exhibit physical properties for practical use, the heat fusible fibers should be used in a major proportion, so that the self-extensibility characteristic of the self-extensible fibers cannot be taken full advantage of.